The metal-semiconductor transition in amorphous Si_1-xCr_xfilms. II. Range of validity of the scaling behaviour of the conductivity, (T,x)= (T/T₀(x)), in the semiconducting region and determination of the minimum metallic conductivity from (T,x) in the metallic region

Abstract

For pt.I see ibid., vol.16, p.6491 (1983). The electrical conductivity sigma (T,x) of amorphous Si1-xCrx films is studied for T=0.1-300K and x=0.09-0.26. The existence of a finite minimum metallic conductivity sigma mm is shown by two independent methods, which yield 270+or-50 Omega -1 cm-1 and 250+or-30 Omega -1 cm-1 respectively. On both sides of the metal-semiconductor transition (with respect to the conductivity scale) only one parameter depending on x and preparation conditions is needed to describe sigma (T) for low T. In particular, on the semiconducting side a scaling law, sigma = sigma (T/T0(x, . . .)), is valid. On the metallic side a contribution beta T1/2 is observed, where beta is completely determined by sigma (0K). The function beta ( sigma (0K)) exhibits a square-root singularity at sigma mm. The critical Cr concentration xc is weakly influenced by the preparation conditions; it amounts to about 0.14. At high temperatures a second conduction mechanism becomes important. This mechanism is present on both sides of the transition. A quantitative description of the related conductivity contribution is given for both the metallic and semiconducting regions. Annealing of semiconducting samples at temperatures of about 300 degrees C or higher destroys the scaling behaviour, presumably due to the formation of metallic clusters. In particular, it gives rise to a decrease of the pre-exponential factor of sigma (T).